The APLS is an autoimmune disease of sporadic and unpredictable thrombosis which leads to miscarriages, venous clots, strokes, and sudden death of young people. The only clinical tests available are nonspecific screening assays which detect a lupus anticoagulant or antiphospholipid autoabs, but do not predict which patients will develop life-threatening disease. Because of this, patients do not receive anticoagulent therapy until after a serious thrombotic event occurs. Then they are left on potentially dangerous anticoagulant regimens for life, with no assurance that they need it. There is an obvious mandate for better predictive testing and for more specific therapies aimed at the underlying coagulation disorder in this syndrome. A critical aspect of the problem that must be addressed by a disease model is the sporadic nature of the thrombotic events that occur. This application will evaluate a likely model for sporadic thrombosis, transient functional deficiency of the anticoagulant protein S. Functional protein S deficiency results from excessive binding by a protein S inhibitor, the C4BP, a dual regulator of the complement and coagulation systems. Functional inhibition of protein S by C4BP has been recently found in patients with APLS by many authors and temporally related to clotting events. The applicants determined that beta2-GPI, the major target antigen for antiphospholipid autoabs, binds protein S and reverses inhibition of protein S by C4BP. A monoclonal anti-beta2- GPI ab inhibits its interaction with protein and causes functional protein S deficiency in vitro. The applicants hypothesize that a subset of pathogenic antiphospholipid autoabs inhibits interaction between beta2-GPI and protein S, causing intermittent thrombosis during states of high complement activity and excess C4BP, such as are found during lupus flare and pregnancy. The applicants will define interactions between protein S, beta2-GPI, and C4BP on a molecular level. Beta2-GPI and C4BP share homologous regions associated with the complement control protein superfamily. These are likely protein S-binding sites and will be targeted in mutagenesis experiments aimed at finding epitopes to which pathogenic antibodies bind. Peptides derived from the protein S-binding region of beta2-GPI might serve as antigens for a more specific clinical assay for pathogenic antiphospholipid autoabs or as a basis for a novel therapeutic agent. The applicants will also evaluate the significance of the sex hormone binding globulin (SHBG) region on protein S. Pregnancy is a risk factor for thrombosis in APLS, and decrease in protein S function is associated with oral contraceptive use. Therefore, possible modulation by sex-hormones of interactions between these proteins will be addressed in this application.